File headers: SPDX License migration

Use a shorter/simpler license convention, stops the header taking so
much space.

Follow the SPDX license specification: https://spdx.org/licenses

- C/C++/objc/objc++
- Python
- Shell Scripts
- CMake, GNUmakefile

While most of the source tree has been included

- `./extern/` was left out.
- `./intern/cycles` & `./intern/atomic` are also excluded because they
  use different header conventions.

doc/license/SPDX-license-identifiers.txt has been added to list SPDX all
used identifiers.

See P2788 for the script that automated these edits.

Reviewed By: brecht, mont29, sergey

Ref D14069

tests/python/cycles_render_tests.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Apache License, Version 2.0
+# SPDX-License-Identifier: Apache-2.0
 
 import argparse
 import os